Viruses have evolved to exploit cellular processes to foster virus replication. A mechanism used by both DNA and RNA tumor viruses is the ligand-independent activation of growth factor receptors. In this competing renewal application, we propose experiments to further our understanding of the interaction of the bovine papillomavirus (BPV) E5 protein and the cellular PDGF beta receptor, and to use the E5 protein as a model to design and select small TM proteins with novel biological activities. This grant supported the experiments that identified the E5 protein as a viral oncoprotein and established its primary mechanism of action as a transmembrane (TM) ligand of the PDGF beta receptor. This grant also supported our detailed analysis of the mechanism of PDGF receptor activation by the E5 protein and our preliminary studies on the structure of the E5/PDGF beta receptor complex. An important advance in the last funding period was our development of a genetic selection to identify small random TM proteins modeled on the viral E5 protein that can activate the PDGF receptor and transform cells. We will extend this work here to isolate novel small TM proteins that bind and activate other receptor tyrosine kinases, with an initial focus on p185neu. The basis for the altered specificity of the small TM proteins will be explored by mutational analysis. In addition, we will use a variety of genetic, biochemical, and computational approaches to generate a detailed molecular model of the E5/PDGF beta receptor heteromeric complex, as a model of specific TM interactions that will provide new insight into the assembly of these important complexes. We will use our collection of E5 and PDGF beta receptor mutants to test our hypothesis that different mechanisms of PDGF beta receptor activation result in different molecular outputs, a situation that could have important implications for the virus life cycle and viral tumorigenesis. Finally, we will explore the contribution of V-ATPase binding to E5 activity. These experiments will not only provide new insights into the action of this interesting and unique viral oncoprotein, but will also provide general insights into cellular signaling and the formation of TM protein complexes. The selection of novel TM ligands modeled on a viral protein may be a breakthrough in attempts to manipulate the structure and activity of proteins in trans, and follows in the long tradition of developing new methods to study and influence cellular behavior by exploiting the biology and strategies of viruses.